1. Field of the Invention
This invention relates to the removal of arsenic from shale oils.
2. Description of the Prior Art
Vast deposits of oil shale, sedimentary marlstone, are known to exist in several areas of the world. Such deposits are found in the United States, with the more commercially important materials located in the states of Colorado, Utah and Wyoming. The geological unit known as the Green River Formation in those states contains oil shale having up to about 85-100 percent by weight of kerogen--a three-dimensional polymer that is insoluble in conventional organic solvents. Upon heating the shale ("retorting"), kerogen decomposes to produce crude shale oil vapors, which can be condensed into a synthetic crude oil and subsequently introduced into a refinery for conversion to valuable fuels, lubricants and other products.
A number of retorting processes are known, generally classified in two categories: "in situ", wherein shale is heated in chambers formed underground without removing a significant portion of the rock material, and "above ground", wherein shale is mined by conventional methods and transported to a pyrolysis device for heating. The various processes each accomplish separation of solid and liquid retort products, using techniques which are specially designed for the particular process.
One successful above ground retorting process is shown in U.S. Pat. No. 3,361,644 to Deering, which patent is incorporated herein by reference. In this process, oil shale is fed upwardly through a vertical retort by means of a reciprocating piston. The upwardly moving oil shale continuously exchanges heat with the downwardly flowing high-specific-heat, recycle gas introduced into the top of the retort at about 1050.degree. F. In the upper section of the retort (the pyrolysis zone), the hot recycle gas educes hydrogen and hydrocarbonaceous vapors from the oil shale. In the lower section (the preheating zone), the oil shale is preheated to pyrolysis temperatures by exchanging heat with the mixture of recycle gas and educed hydrocarbonaceous vapors plus hydrogen. Most of the heavier hydrocarbons condense in this lower section and are collected at the bottom of the retort as a product oil. The uncondensed gas is then passed through external condensing or demisting means to obtain additional product oil. The remaining gases are then utilized as a product gas, a recycle gas as hereinbefore described, and as a fuel gas to heat the recycle gas to the previously specified 1050.degree. F. temperature.
In all known oil shale retorting processes, arsenic components of the shale either sublime to or are pyrolyzed into vaporish arsenic-containing components. As a result, arsenic in various forms collects with the educed hydrocarbonaceous vapors and condenses with the higher molecular weight hydrocarbons in the preheating zone or, in some processes, in a condenser situated outside of the retorting vessel. When oil shale from the Green River Formation is retorted, the concentration of arsenic in the , produced crude shale oil is usually in the range from about 25 to about 100 parts per million by weight (ppmw)
Shale oil can be refined to produce valuable products, lubricants and the like, using similar methods to those known for petroleum processing, such as catalytic cracking, hydrotreating, hydrocracking, and others. Problems arise, however, due to irreversible poisoning of expensive catalysts during shale oil processing. This poisoning is caused by the arsenic in the shale oil, which is generally present in a different form and in a far greater proportion than ordinary petroleum feeds.
In addition to causing processing difficulties, the arsenic content limits the usefulness of shale oil even in its unrefined state, since burning high arsenic-containing fuels results in unacceptable pollution. For these reasons, it is desirable to reduce the amount of arsenic present in shale oils to the lowest possible level. Young, in U.S. Pat. Nos. 4,046,674 and 4,075,085 describes methods to remove arsenic utilizing a solid absorbent containing nickel and molybdenum on refractory oxides ('674) and oil-soluble nickel, cobalt or copper-containing additives ('085). Furthermore, Albertson, in U.S. Pat. No. 4,446,006 describes a process for removing arsenic by adding elemental sulfur or aqueous sodium hydrogen phosphate to a shale oil.
A need remains for a simple, inexpensive method for reducing the arsenic content of shale oils. Accordingly, it is an object of the present invention to provide a method for removing arsenic from shale oil or fractions thereof. A further object is to remove arsenic upstream of solid arsenic-removing absorbents or hydrotreating catalysts deactivated by arsenic so as to extend their lives. These and other objects of the invention will become more apparent in view of the following description of the invention.